A system for transmitting luminance and chrominance without interaction through a PAL channel has been proposed, known as Weston Clean PAL and described in GB-A-2044577 and GB-A-2113037 the disclosures of which are incorporated herein by reference. This system obtains separation of luminance and chrominance by making them look as if they are effectively in phase quadrature over the chrominance region before manipulating them to look like a PAL signal. This separation process is discussed in more detail in "A Compatible Improved PAL System" published in the EBU review, February 1986. This is achieved by sampling the luminance at twice the subcarrier frequency, so making it appear as double-sideband modulation of the subcarrier in one phase and, at the same time, forming the U and V chrominance into a single signal which modulates the quadrature phase of the subcarrier. This phase separation is effective so long as both sidebands of both phases are equal. If they are not, then cross-talk between luminance and chrominance will occur at the frequencies where the inequality occurs. As the video signal is limited in bandwidth it therefore follows that the bandwidth of the chrominance signal is limited to the difference between the maximum video frequency and the subcarrier frequency. For system I PAL this is 1.1. MHz, for a system BG PAL this is approximately 0.57 MHz.
In the simplest case with system I PAL, where only vertical filtering is used before combining the U and V signals into a single signal, the vertical bandwidth of the U and V signals is 72 cycles/active picture height (c/aph). On the other hand, the video bandwidth limit of 1.1 MHz corresponds to a horizontal bandwidth of 57 cycles/active picture width (c/apw). Bearing in mind the aspect ratio of the picture, which may be 4:3 or 16:9, it is apparent that the horizontal bandwidth limiting imposes a disparity between horizontal and vertical resolution which becomes substantial for a 16:9 aspect ratio. The spectrum of the combined chrominance is shown in FIG. 1 and is scaled assuming an aspect ratio of 16:9.
With more complex prefiltering of the U and V signals before combination, the vertical bandwidth can be increased at the expense of temporal bandwidth as described in "A Compatible Improved PAL System", published in the EBU Review, February 1986. In such a situation the horizontal/vertical disparity would then be further increased.
We have appreciated that it is possible to overcome this disparity by spectrum folding the U and V signals since the Weston system makes no assumption about the horizontal sampling of the signals. Spectrum folding is a known technique and allows spectral components with high horizontal frequency and low vertical and/or temporal frequency to be carried as components with low horizontal frequency and high vertical and/or temporal frequency, provided the signals are appropriately pre-filtered. It therefore exchanges certain spectral regions for others. The technique was first applied to component signals by Golding and Garlow in 1971 and subsequently to PAL signals by Phillips and Weston in 1974. In these applications a modest amount of folding allowed a video bandwidth of 20-30% above the channel bandwidth to be sent. However, by sacrificing more resolution in the vertical and/or temporal dimensions it is possible to double or even quadruple the horizontal bandwidth by an appropriate folding.
The invention is defined by the claims to which reference should now be made.
The invention is based on the appreciation that spectrum folding can be applied in the particular context of the chrominance channel of a Weston Clean PAL system. This means that there are certain constraints on the nature of the folding frequencies that can be applied since such folding requires the U and V video signals to be sampled with an appropriate structure whose vertical and temporal parts must also be compatible with the basic Weston Clean system. Such an operation may be regarded as preceding or being part of the combination of the U and V signals.
Preferably, the samples are turned into a continuous analogue signal by low pass filtering with a so-called "Nyquist" filter which cuts at half the video sampling frequency, proportional to the horizontal component of the sampling structure. As this cut frequency must be in the region of 1.1 MHz for system I PAL it, in turn, governs the horizontal component of the sampling structure. The analogue signal is then transmitted via the modulation and demodulation of the subcarrier in the normal way. The original samples can then be recovered from the transmitted signal by resampling it with the same structure and the samples must be post filtered to remove alias components, yielding U and V signals with increased horizontal bandwidth. Again this process can be considered as following or being part of the separation of the U and V signals.